The present study was designed to secure the information about the mechanism of photoperiodic perception that discriminates between light and darkness and the ecophysiological meanings of daily change in night-length, using a strongly photoperiodsensitive rice cultivar, Norin 18.1. Effect of twilight at dusk -- During evening twilight on cloudless days, 15 - 20 min had elapsed while the decrease in natural illuminance from the twilight values of 400 - 500 lux to about 30 lux. Therewith, the ratio of far-red (FR) to red (R) light increased from values of about 1.0 to those in the range of 1.2 to 1.4. This decreased illuminance and the increased ratio of FR/R could have natural end-of-day signals coupling to the beginning of photoperiodic dark-time measurement.2. Experimental approaches with artificial lights -- 1)Dusk-to-dawn lighting of R light at 0.2lmumol/m^2/s could not induce to flower and that of incandescent white light of 30 lux (6 mumol/m^2/s) and above was also the case, sugge
… Moresting that those irradiance of artificial light failed to act as a dark signal. 2)Exposing plants to incandescent light of low illuminance before entry to non-inductive night close to the critical, flowering did not take place, if removed FR radiation. But the same light of about 30 lux, when contained FR region of the spectrum, was found to be equivalent to photoperiodic darkness. Enriching FR radiation with lowering voltage, without changing the illuminance, the flowering response was enhanced. Incandescent light, even of 300 lux, of which FR/R ratio exceeded 2.0, was effective to promote flowering.3. Daily change in night length -- Flowering of plants of younger age was favored under lengthening night conditions. On the other hand, shortening dark periods were favorable for flowering of plants of fully responsive age to photoperiod.We concluded from these observations that the light/dark transition at dusk is sensed by reduction in light quantity as well as by increase in FR/R ratio and that the effects of gradually changing night-length on flowering of rice are dependent on the developmental age of plants. Less